This invention relates to an apparatus and method to allow an active matrix liquid crystal display to be viewed effectively in either daylight conditions or with the aid of night vision electronic viewing equipment under dark conditions.
Active matrix liquid crystal displays are presently being developed and used widely as instrument monitors in many fields of use. Particularly, it is advantageous to make use of flat liquid crystal displays in the cockpit instrumentation panel where the status of various sensors are displayed for viewing by the pilot. In the past, such displays contained cathode ray tubes (CRT) to convert the electronic information to visual pictures of the data viewable by the pilot. CRT displays require considerable volume for their installation. The advantage of any flat-panel displays such as active matrix liquid crystal displays is obvious.
Liquid crystal displays can be constructed as multi and full color displays in several ways. One way is to use fluorescent lamps excited by ultraviolet radiation as a backlighting panel. The light from this bright flat-panel is modulated by the LCD to create the patterns of information bearing pictures on the display for viewing by the operator. The backlighting panel can be a fluorescent white lamp with proper filters or of a properly organized blue-green-red matrix fluorescent phosphor structure.
Several types of fluorescent lighting for LCD backlighting have been developed and used. One type, as an example, is a flat fluorescent lamp similar to that reported in the article "Flat Fluorescent Lamp for LCD Back-light" by K. Hinotani, S. Kishimoto, and K. Terada, 1988 Society for Information Display, International Display Research Conference, pp. 52-55. Hinotani et al shows the construction of a flat-panel fluorescent lamp designed for use in small size portable LCD televisions.
U.S. Pat. No. 4,842,378 by Flasck et al presents a method of illuminating flat panel displays utilizing a serpentine shaped fluorescent tube which is placed adjacent to the back screen of the panel to backlight the screen. Also shown in this patent is the use of individual fluorescent tubes spaced parallel to one other along a back plane to create the backlighting for flat-panel display purposes.
U.S. Pat. No. 4,748,546 by Ukrainsky presents a similar use of fluorescent tubes where such fluorescent tubes are embedded in RTV rubber or other transparent potting material, and with the use of diffusing layers becomes a backlighting device for a flat panel display.
In addition to the use of fluorescent tubes other methods of providing light sources have been suggested. The U.S. Pat. No. 4,573,766 by Bournay, Jr. et al presents a light emitting diode (LED) backlighting panel with the LED light sources installed in side edges of the solid state panel. The panel then serves as a wave guide to distribute the light along a roughened surface perpendicular to the edges where the LED's are installed. At the roughened surface the light is diffused upward through the LCD and display area.
Each of these methods of lighting a flat-panel display, and others not mentioned here, provide the capability and facility of emitting light at high intensity quite suitable for daylight viewing. In many cases, particularly military uses, it becomes necessary to be capable of viewing the same display during dark conditions where the viewer is utilizing night vision electronic equipment. Such night vision electronic equipment usually is designed to be sensitive to very low light level intensities, frequently in the near infrared region.
Any near infrared source of light at an intensity above the surrounding night time conditions will have the tendency to cause night vision electronic viewing equipment to overload and cease functioning. Quite clearly then, the use of LCD displays designed for daylight use become a handicap at night in an aircraft where a pilot is relying on night vision equipment to view the environment about him. It became necessary, therefore, to conceive and develop a method whereby the intensity of light emitted by the LCD display in the near infrared spectrum is sufficiently low as not to interfere with the operation of night vision equipment and would thus allow the operator the ability to view such display through the night vision equipment.
One possible solution to this problem is the design of an optical filter for the display surface which would allow sufficient visible throughput for daylight observation while maintaining sufficient suppression of the transmission of wavelengths that night vision equipment are susceptible to, e.g. infrared (IR). Several studies were conducted by the inventor on candidate filters for such a purpose. Unfortunately, the results revealed that those filters which would reduce the IR portion of the spectrum simultaneously did not have the best visible transmission. Therefore, with the reduced visible transmission, operation under daylight conditions was handicapped by the reduced brightness of visible light available at the display.
It is an object of this invention to present an apparatus and method for converting a standard fluorescent activated backlit display suitable for daylight viewing into a simultaneous display capability of emitting very low-level light intensity during dark viewing conditions with night vision equipment.
It is a further object of this invention to provide, by the addition of a secondary matrix of fluorescent light sources, or either light emitting diodes or miniature low-level incandescent lamp light sources, the ability to convert a standard fluorescent backlit active matrix liquid crystal display into a display capable of effective viewing during daylight conditions with the unaided eye and during dark night conditions with the observers eyes augmented by night vision electronic equipment.